Sample.java

package visibility;

public class Sample {
    public static void main(String[] args) {
        final User user = new User("Neto");

        User.changePrivateName(user, "João Batista Neto");

        user.showPrivateName(); // João Batista Neto
    }
}

User.java

package visibility;

public class User {
    private String privateName;

    public User(String name) {
        privateName = name;
    }

    public static void changePrivateName(User user, String name) {
        user.privateName = name;
    }

    public void showPrivateName() {
        System.out.println(privateName);
    }
}